1. Field of the Invention
This invention relates to motor vehicle antennas and more particularly to a multiband AM/FM/cellular telephone antenna system suitable for use on automotive vehicles.
2. Description of the Related Art
In recent years with the introduction of cellular telephone systems, the use of telephones in motor vehicles has increased dramatically. The cellular telephone system typically requires a separate antenna. On automobiles, the cellular antenna may be factory-installed original equipment or subsequently installed after-market equipment. The cellular telephone antenna is typically a rear window mounted antenna or a trunk lid antenna. The window mounted antenna is preferred since it may be mounted at a higher location than the trunk lid. The window mounted antenna extends above the roof of the passenger compartment to gain height, thereby extending operating range and to avoid the blocking of signals by the passenger compartment.
Particularly for after-market equipment, the installation of the necessary wiring from the front of the car to the rear window mounted antenna, preferably hidden from view behind the roof headliner, is a time-consuming job which adds substantially to the cost of installation of the cellular telephone in the vehicle.
Since practically all cars, trucks and the like are equipped with an AM/FM radio, more and more such vehicles now have two antennas, one for the AM/FM radio and another for the cellular telephone. The AM/FM antenna is almost always a rod antenna. The cellular telephone antenna typically includes a helically wound coil near the middle of the antenna. The coil improves the effectiveness of the antenna by impeding phase reversal, but detracts from the overall appearance. Furthermore, a well-known problem with the cellular telephone antenna of that type is the wind noise or whistle caused by the coil when the vehicle travels at highway speeds.
Cellular phone antennas are typically rod antennas and mounted on or adjacent a ground plane area, such a trunk lid or passenger compartment roof, since a substantial ground plane is necessary for effectiveness of the typical rod antenna. Cellular phone rod antennas are generally not mounted on vehicle fenders, as are AM/FM antennas, since the fender typically drops off nearly vertically and does not always provide the desired ground plane at cellular phone frequencies.
Portable vehicular cellular telephones offer flexibility in that they may be taken from vehicle to vehicle. A serious drawback of these portable cellular phones is that their range is limited. This is due in large part to the limitations of the antenna of the portable units. The portable vehicular cellular telephones are typically provided with an inefficient, flexible whip antenna referred to as a "rubber ducky." When they are used with such an antenna in cars or trucks and the like, the range of these units and their usefulness is limited due to the ineffective antenna and due to signal blocking by the metallic vehicle enclosure.